Transparent windows are employed in a wide range of defense and commercial applications, including optical lenses and photovoltaic cover glass. Glass, sapphire, and quartz are well-known materials used to form high transmittance optical windows for a wide range of applications. Because these materials have very low absorption coefficients over a wide range of photon energies, optical transmittance through glass, sapphire, and quartz windows is typically limited by reflection losses. In particular, Fresnel reflection losses in optical windows arise from the difference in index of refraction between air (n˜1) and the window material (n˜1.4-1.8). Although Fresnel reflection losses are typically relatively low at normal incidence, they can become quite substantial for off-angle light incidence. For example, Fresnel reflection from uncoated glass generally varies from over 4% at normal incidence to as much as 40% at an incident angle of 75°.
Reducing optical reflection from surfaces is highly desirable to many applications in optics. Reducing reflection is commonly achieved through coating or texturing the surface of interest. Numerous applications involving dielectric or semiconducting materials use the light that is transmitted through the material's surface. Examples of such an application are optical lenses, windows, photovoltaic devices, display devices, and photo-detectors. Glass (amorphous SiO2) is an example of a dielectric material widely used in a variety of optical applications (e.g. lenses, windows) and as a front sheet for semiconductor optoelectronic devices.
Theoretically, it has been known for some time that Fresnel reflection losses can be minimized between two media by varying the index of refraction across the interface. Until recently, however, the unavailability of materials with the desired refractive indices, particularly materials with very low refractive indices, prevented the implementation of step-graded refractive index designs.